In many applications, an electronic device needs to survive extreme external forces. This can range from devices being dropped on the floor, to sustaining impact from various projectiles in harsh environments that may include high speed collisions with sharp, solid debris which are characteristic of combat, high-speed, and catastrophic environments. Special effort has to be paid to the design and construction of electronic devices used in these applications, since construction materials deform under such external forces, and to ensuring continued survival and maintaining functional usage of such devices when placed in such environments.
There are different types of deformations. In “elastic” deformation, materials will return to their original states after the forces are removed. In “plastic” deformation, the change is irreversible. As its name suggests, plastic materials have a wide plastic deformation range. Another category of materials with a large plastic deformation range is ductile metals.
For the most popular display substrate material, e.g., glass, the mechanical failure under high external forces is “Fracture”. Glass shatters when a large external force is applied. For example, a ½ pound steel ball dropped from 3 feet generates about 2 Joules of kinetic energy. This generally causes breakage of ¾″ thick normal glass plate. This is described as Case A in Table 1 below. Table 1 illustrates four impact cases involving various masses, velocities and kinetic energies.
TABLE 1massvelocityKinetic energy(gram)(m/s)(J)A2274.232B22714.6724C10463206D10479.5329
To improve the impact resistance, glass can be toughened (i.e., tempered) through thermal and chemical treatment. The basic principle is to introduce compressive stress at the glass surfaces. With the treatment, treated glass becomes much stronger. For example, the toughened glass with the same thickness (e.g., ¾″) can withstand the same steel ball dropping from 36 feet with 12 times more kinetic energy, as shown as Case B in Table 1.
To further improve the impact resistance, traditional “bullet” proof glass can be used. However, it is not preferred to use traditional bullet proof glass as display substrate material because of optical effects. Traditional bullet proof glass may include many alternate layers of glass, air pockets, and thermoplastic material. The glass layers closer to the impact surface can easily get damaged. Although this effectively slows down a bullet, it does generate many cracks, which may not be acceptable for optical reasons in certain cases because the densely distributed cracks will make the image not recognizable.